In compressed air supply devices of motor vehicles, it is known how to measure the relative humidity at the outlet of the air dryer or in a compressed air tank by means of a moisture sensor designed as a moisture switch, the output signals of which are supplied to evaluation/control electronics. In the event that the relative humidity exceeds a threshold value, the evaluation/control electronics initiate regeneration of the dehumidification device or generate a warning signal to indicate, for example, that the dehumidification device has greatly deteriorated in terms of efficiency and/or that it must be replaced. As an improvement of the moisture switch, capacitive moisture sensors, in which a hygroscopic polymer layer, for example, is disposed as a dielectric between two capacitor electrodes, are, among other options, also used. Depending on the ambient water vapor pressure, more or less water is absorbed in the polymer layer. Thereby, the dielectric constant and, in turn, the capacitance of the capacitor changes. As is known, the compressed air in commercial vehicles is contaminated with different constituents of the intake air, some harmful for the polymer layer, and with harmful oil and oil-decomposition products from the compression process in the compressor. As a result, the effectiveness of the capacitive principle is greatly impaired by such contaminants after a prolonged exposure time, thus, in particular, reducing the accuracy and shortening the useful life of the moisture sensor.